Luminescence emission occurs after an appropriate material has absorbed energy from a source such as ultraviolet or X-ray radiation, electron beams, chemical reactions, and so on. The energy lifts the atoms of the material into an excited state, and then, because excited states are unstable, the material undergoes another transition, back to its unexcited ground state, and the absorbed energy is liberated in the form of either light or heat or both (all discrete energy states, including the ground state, of an atom are defined as quantum states). The excitation involves only the outermost electrons orbiting around the nuclei of the atoms. Luminescence efficiency depends on the degree of transformation of excitation energy into light, and there are relatively few materials that have sufficient luminescence efficiency to be of practical value.
The name luminescence has been accepted for all light phenomena not caused only by a rise of temperature. With respect to organic molecules, the term phosphorescence means light emission caused by electronic transitions between levels of different multiplicity, whereas the term fluorescence is used for light emission connected with electronic transitions between levels of like multiplicity. Although the inorganic phosphors are industrially produced in far higher quantities (several hundred tons per year) than the organic luminescent materials, some types of the latter are becoming more and more important in special fields of practical application. Paints and dyes for outdoor advertising contain strongly fluorescing organic molecules. Their main shortcoming is their relatively poor stability in light, because of which they are used mostly when durability is not required. Organic phosphors are used as invisible markers of banknotes, identity cards, and stamps and for fluorescence microscopy of tissues in biology and medicine. Their “invisibility” is due to the fact that they absorbpractically no visible light. The fluorescence is excited by invisible ultraviolet radiation (black light). The practical value of luminescent materials lies in their capacity to transform invisible forms of energy into visible light.
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